Demand for wireless communication products increases exponentially every year. As the demand increases, frequency bands shift to the higher side since it is not possible to provide for all the demands at a given frequency band. Currently, there are many wireless communication products that operate in the 900 Mhz, 1.9 GHz, and 2.5 GHz bands. The operating frequency of Personal Communications System (PCS) including cellular is in 824 to 5850 MHz in six non-continuous bands.
As operating frequency increases, semiconductor device geometry has to shrink. A 1.0 micron gate length must shrink to 0.5 microns or less as frequency increases. A shorter gate length means faster electron transit time under the gate, which is necessary for high frequency operation. However, gate (metal) resistance increases as the gate length is reduced, thereby degrading device performance (such as gain and noise figure). This degradation becomes severe as operating frequency increases.
There are a variety of different techniques for reducing gate resistance. FIG. 10 depicts a T-gate 24 where the bottom 26 of the "T" defines the reduced gate width and the top 28 of the "T" is large to prevent increasing the gate resistance. Other techniques are the Y-gate and mushroom gate configurations.
However, these techniques require complicated processing techniques or advanced processing equipment. For example, one technique requires eleven process steps including deposition and etching of nitride layer to form a gate recess.
An etch-back process used in silicon processing (deposit dielectric film, define gate by photolithography, and etch film) requires critical dimension control twice: 1) in photoresist opening, and 2) during dielectric film etching.
The Self-Aligned Implantation for n+- Layer Technology (SAINT) developed by NTT, Japan uses a silicon oxide lift-off technique where the photoresist defining gate sits on silicon nitride. In SAINT the gate contact region is opened by etching silicon nitride in reactive ion etching (RIE) using silicon oxide as an etching mask. Thus, the SAINT technique has the same drawbacks as the etch-back process described above.